The very earliest cloud height measurements utilized ceiling balloons of various weights and lifts, inflated with helium. The time interval between the release of the balloon and its entry into the base of the clouds was recorded. The point of entry for layers aloft was considered as midway between the point at which the balloon began to fade and that when it completely disappeared. With surface based clouds, the time interval ended when the balloon completely disappeared. During the day, red balloons were used with thin clouds and black balloons were used with thicker clouds and at night, a battery-powered light was attached. Naturally, accuracy using this method depended somewhat on the reactions and eyesight of the observer and could be complicated by such issues as wind and local topography.
In the 1930s, a methodology originated whereby a beam from a ceiling light was projected at a 45-degree elevation into the sky. The projector was rotated about the vertical axis until the light beam hit the lowest cloud. An observer paced off the distance from the projector to a point directly below the illumination spot. With the geometry of this scheme, the paced distance equaled the height of the cloud. This technique was quickly abandoned in favor of a vertically shining light with a clinometer at a previously measured baseline. Knowing the baseline length and elevation angle in this right triangle situation made it easy to determine the height with a lookup table.
Much of the human introduced subjectivity was later removed by automation using a photocell that scanned the vertical path until the spot of light on the cloud was detected. The projector light was modulated so the photocell received less interference from ambient light during daytime use. The angle of inclination was displayed automatically at the observer's console.
The next version of cloud height indicators (CHI) was the rotating beam ceilometer. As the name implies, the beam of light rotated, and the vertically looking detector measured any cloud hits directly overhead. The angle of the cloud hits was displayed either on a scope or on a recorder chart. Height measurements were limited to heights no greater than ten times the base line. Above this ratio, the value of the tangent function increased too quickly to ensure the accuracy of a measurement.
Light detection and ranging (LIDAR) is a method that can be used to characterize the atmosphere, and many methods have been developed to produce LIDAR systems for specific applications. A LIDAR system usually includes a light transmitter, a receiver—including optics and an electronic light detection device, and some type of timing circuitry. LIDARs for cloud ceiling measurement began service with the National Weather Service (NWS) CHI service in 1985. The sensor sends laser pulses vertically into the atmosphere. The pulse rate varies with the temperature to maintain a constant power output. The time interval between the pulse transmission and the reflected reception is used to determine the cloud height. The reporting limit of this instrument for the NWS is 3800 m (12,000 ft).
Different configurations for the transmitting and receiving optics have been developed over the years including the bi-axial or side-by-side configuration, where the transmitter and receiver optics are separate but adjacent. This produces problems in overlap of the transmitting beam and the receiving field of view for certain ranges. The advantage, however, is that the full power of the laser is transmitted, and the full return power from atmospheric backscattering is received by the receiving optics. In a coaxial configuration, the transmitting and receiving optical pathways share the same centroid. For full overlap of the transmitting and receiving optical ray traces, some type of beam splitter is usually employed resulting in up to a 75% loss in optical power due to the two passes through a 50/50 beam splitter, for example. Additionally the receiving field of view includes the optical surfaces where the transmitter light exits. Reflections from these surfaces can result in excessively high power at the receiver, disrupting measurements.
Other configurations have been introduced using a beam-splitting mirror with a hole in it, creating a coaxial configuration, where the transmitting beam suffers no loss on the outgoing path, and the receiving beam is less contaminated with crosstalk from the transmitting beam reflecting off of the outgoing optics. However, while crosstalk is reduced, it is not eliminated. Multiple reflections within the lens can bring light back from the transmitter to the receiver.
Needs exist for improved methods of measuring cloud heights.